Did you get the part that ripped off out from under the slide?
That plastic piece is a unsealed corner cover, that is held in place with the screw track you see.
You will need to find out what ripped it off (staple, screw or from sitting, a goob of sealant)
You will need a new corner cover and some butyl tape for the repair. The only PITA will be the screw that is behind the slide seal but you can reach that last screw from the inside with the slide brought in a bit.
Also make sure you slide box is centered in the hole, it might be binding against the wall on that side?